The present invention relates generally to wireless radio telecommunication. More specifically, the invention relates to downlink (DL) power control from a base station (BS) to a mobile station (MS or UE) in a cellular telecommunications network.
In a cellular communication system, a mobile radio station (MS) communicates over an assigned radio channel or link (RL) with a radio base station (BS). Several geographically-dispersed base stations are connected via digital transmission links to a switching node which is typically connected to a gateway that interfaces the cellular communications system with other communication systems. A call placed from an external network to a mobile station is directed to the gateway, and from the gateway through one or more switching nodes to a base station which serves the called mobile station. The base station pages the called mobile station, and assuming the mobile station responds to the page, establishes a radio communications channel. A call originated by the mobile station follows a similar path in the opposite direction (although there is no need for a page for a mobile-originated call).
In a code division multiple access (CDMA) mobile communications system, spreading codes are used to distinguish information associated with different mobile stations or base stations transmitting over the same radio frequency band. In other words, individual radio xe2x80x9cchannelsxe2x80x9d correspond to and are discriminated on the basis of these spreading codes. Spread spectrum (e.g., CDMA) communications permits mobile transmissions to be received at two or more (diverse) base stations and be processed simultaneously to generate one received signal. With these combined signal processing capabilities, it is possible to perform a handover from one base station to another, or from one antenna sector to another antenna sector connected to the same base station, without any perceptible disturbance in the voice or data communications. This kind of handover is typically called xe2x80x9cdiversity handover.xe2x80x9d Diversity handover may include xe2x80x9csoftxe2x80x9d and xe2x80x9csofterxe2x80x9d handover. During diversity handover, the signaling and voice information from plural sources is combined in a common point using decisions made on the xe2x80x9cqualityxe2x80x9d of the received data. In soft handover, as a mobile station moves to the edge of a base station""s cell, the adjacent cell""s base station assigns a transceiver to the same call while a transceiver in the current base station continues to handle that call. As a result, the call is handed over on a xe2x80x9cmake-before-break basis.xe2x80x9d Soft diversity handover is therefore a process where two or more base stations handle a call simultaneously. xe2x80x9cSofterxe2x80x9d diversity handover occurs when the mobile station is in handover between two or more antenna sectors connected to the same multi-sector base station using a similar make-before-break methodology.
Because all users of a CDMA communications system transmit information using the same frequency band at the same time, each user""s communication interferes with communications of other users. In addition, signals received by a base station from a mobile station close to that base station are much stronger than signals received from other mobile stations located at the base station""s cell boundary. As a result, distant mobile communications may be over-shadowed and dominated by close-in mobile stations which is why this condition is sometimes referred to as the xe2x80x9cnear-far effect.xe2x80x9d
The physical characteristics of a radio channel vary significantly for a number of reasons. For example, the signal propagation loss between a radio transmitter and receiver varies as a function of their respective locations, obstacles, weather, etc. As a result, large differences may arise in the strength of signals received at the base station from different mobiles. If the transmission power of a mobile station signal is too low, the receiving base station may not correctly decode a weak signal, and the signal will have to be corrected (if possible) or retransmitted. Accordingly, erroneous receipt of signals adds to the delay associated with radio access procedures, increases signal processing overhead, and reduces the available radio bandwidth because erroneously received signals must be retransmitted. On the other hand, if the mobile transmission power is too high, the signals transmitted by the mobile station create interference for the other mobile and base stations in the system. Ideally, all mobile-transmitted signals should arrive at the base station with about the same average power irrespective of their distance from the base station.
Interference is a particularly severe problem in CDMA systems because large numbers of radios transmit on the same frequency. If one radio transmits at a power output that is too large, the interference it creates degrades the signal-to-interference ratio (SIR) of other received signals, making it more difficult to correctly demodulate those signals. Accordingly, transmit power control (TPC) is important in a CDMA system. In uplink (UL) transmit power control, the mobile station attempts to control its transmit power to the base station based on the power control messages sent to the mobile station from the base station with the goal of controlling the power level of the signals received at the base station within a relatively small tolerance, e.g., 1 dB, for all mobile station transmissions received at that base station. In downlink (DL) power control, a focal point of this invention, the base station varies the power it is transmitting to a mobile station depending on transmit power control messages or TPC commands sent by the mobile station.
A problem with downlink power control is that the single transmit power control (TPC) command sent from the mobile station to all of the base stations involved in the diversity handover is not received identically. Because there are different paths between the mobile station and each of the base stations, and because different conditions affect each of those paths, the commands received at different base stations have different bit errors. As a result, the TPC command may be received correctly in one base station and incorrectly in another base station. The result is that the average transmit powers of the base stations involved in the diversity handover (which should either be the same or have a fixed offset) begin to drift away from the desired value(s). As this base station power drift increases, the full diversity gain is not realized. Diversity gain is ideally realized by receiving two or more radio links of equal power. If one link has a higher power than needed, the extra power is interference which decreases the overall capacity of the communications system. If one link has a lower power than it should, there is a loss of diversity gain.
To combat base station power drift, the power transmission level of each base station in a diversity handover may be compared to a power reference (Pref) established for all base stations in the diversity handover. The power reference may be set individually per connection (e.g., UTRAN-UE connection) or per radio link in certain embodiments. The difference between measured transmit power (Pinit) of each base station and the reference power Pref may then be used to correct or balance the transmit power level of that base station. The reference power level(s) used in compensating for base station power drift is(are) advantageously determined using one or more parameters relevant to the current condition of the diversity handover communication. Rather than setting an arbitrary and static reference power level, the reference power level may be set dynamically so that it is relevant to the current conditions of the diversity handover communication. Dynamic and adaptive reference power level setting results in more effective and more efficient downlink power control. Exemplary reference power values Pref and exemplary methods of obtaining the same are discussed in commonly owned U.S. Ser. No. 09/531,650, filed May 31, 2000, the disclosure of which is hereby incorporated herein by reference. For example, an average transmit power may be determined for first and second base stations, and the reference power level Pref set to a mean of the average power levels of the first and second base stations. In another example, the average transmit power of a dominant or favored base station may be calculated and used as the reference power Pref.
3G TS 25.214 (V3.3.0) (2000-06), the disclosure of which is hereby incorporated herein by reference, describes a system and method for DL power control. In general, a MS (or UE) generates TPC commands to control the network transmit power. These TPC commands are sent by the MS to base station(s) in the TPC field of the uplink DPCCH (Dedicated Physical Control Channel). Upon receiving the TPC commands, the UMTS terrestrial radio access network (UTRAN) adjusts its downlink (DL) power accordingly. As described in TS 25.214, Section 5.2.1.2.2, after estimating the kth TPC command, the UTRAN adjusts the current downlink power P(kxe2x88x921) [dB] to a new power P(k) [dB] according to following equation:
P(k)=P(kxe2x88x921)+PTPC(k)+Pbal(k)xe2x80x83xe2x80x83(1)
where PTPC(k) is the kth power adjustment due to the inner loop power control, and Pbal(k) is a power balancing correction according to a DL power control procedure for balancing radio link (RL) powers toward a common reference power. The instant invention relates to how to determine and/or implement Pbal(k).
Annex B.3 of 3G TS 25.214 discusses an algorithm for calculating Pbal(k). Unfortunately, this algorithm is somewhat exponential in nature in that it causes large valued corrections Pbal(k) to be made early on in the DL power control process which are followed by smaller changes in DL power control. It is believed that initial large power control changes on the DL may be detrimental and adversely affect the inner loop power control.
Accordingly, it will be apparent that there exists a need in the art for a system and corresponding method for DL power control balancing which can be carried out in a more distributed manner. In other words, there exists a need for a DL power control balancing correction(s) that can be implemented through a number of smaller balancing correction or adjustment steps spread out over a plurality of spaced apart slots on the DL.
Consequently, the present invention includes a power control system and/or method for controlling the downlink (DL) transmit power from a base station (BS) to a mobile station (MS) in a cellular telecommunications network. A power balancing value xcex94P may be divided into a number of smaller correction implements which are distributed over a plurality of spaced apart slots for DL power control. For example, if it is desired to correct DL transmit power by six dB for balancing purposes, a total of six different correction values of one dB each may be implemented with adjacent ones of the correction values being spaced apart by a number of slots. For example, one of the single dB corrections may be implemented to control DL transmit power every ten slots so that the six corrections of one dB each are spread out over a total of sixty slots. The distribution or spreading of DL transmit power corrections over a larger period of time results in a plurality of smaller corrections as opposed to one or a few large corrections, thereby reducing potential adverse effects on inner loop power control of the network.